1. Field of the Invention
The present invention describes a formulation that when mixed with water to produce a self-leveling slurry quickly hydrates to form a high strength floor underlayment. The formulation is characterized by the inclusion of calcined gypsum of the beta (β) form, i.e., beta calcium sulfate hemihydrate (or beta gypsum), made by a continuous calcination process.
2. Description of Related Art
U.S. Pat. No. 4,661,159 describes a composition for producing a floor underlayment that includes an alpha calcium sulfate hemihydrate (alpha gypsum), Portland cement, fly ash, and a beta calcium sulfate hemihydrate (beta gypsum). The composition may also include a small amount of a retarder, defoamer, and superplasticizer and is mixed with water and sand to produce a pourable, self-leveling and quick-setting slurry. A preferred formulation is said to contain 45%-55% by weight beta gypsum; 20% to 30% by weight alpha gypsum; about 5% fly ash by weight; and about 20% Type II Portland Cement by weight. The method by which the beta gypsum is prepared is not discussed.
In U.S. Pat. No. 5,424,099 the combination of a hydraulic cement, 10 to 20% by wt., such as Portland cement, alumina cement, fly ash, blast furnace slag and silica fume; alpha calcium sulfate hemihydrate (alpha gypsum) 70 to 90% by wt., along with a superplasticizer is purportedly used as a self-leveling floor underlayment. Exemplary superplasticizers include naphthalene sulfonates; naphthalene sulfonate-formaldehyde condensates; calcium lignosulfonate; melamine sulfonate-formaldehyde condensates; and polycarboxylic acids.
U.S. Pat. No. 5,439,518 describes a composition based on a lime-containing fly ash that also can be formulated for floor underlayment applications. In the floor underlayment application, the ingredients include about 10-89 wt. % of a lime-containing fly ash, preferably 45 to 54 wt. %; about 10-89 wt. % hydratable gypsum, preferably 45-54 wt. % and at least about 0.05 wt. % of an additive selected from: a setting retarder, dispersant, and/or pigment. The patent suggests that alpha gypsum hemihydrate and beta gypsum hemihydrate can be used interchangeably without significantly impacting the properties of the resulting set product, although alpha hemihydrate is preferred from a commercial perspective. Again, the method by which the beta gypsum is prepared is not discussed.
In U.S. Pat. No. 5,685,903 and U.S. Pat. No. 5,718,759 a composition, also identified as being useful for a floor underlayment, is described containing about 20 wt. % to about 75 wt. % calcium sulfate beta-hemihydrate (beta gypsum), preferably 30 to 50 wt. %; about 10 wt. % to about 50 wt. % Portland cement, preferably 6 to 25 wt. %; about 4 wt. % to about 20 wt. % silica fume, preferably 4 to 8 wt. %; and about 1 wt. % to about 50 wt. % pozzolanic aggregate, preferably 1 to 15 wt. %. The Portland cement component may also be a blend of Portland cement with fly ash and/or ground blast slag. Nothing is disclosed about the source of the beta gypsum.
More recently, U.S. Pat. No. 7,056,964 proposed a composition useful for preparing a floor underlayment containing about 50 wt. % to about 98 wt. % calcium sulfate hemihydrate, at least 25% being calcium sulfate beta-hemihydrate (beta gypsum); about 0.05 wt. % to about 50 wt. % of an enhancing component, such as Portland cement, fly ash, blast furnace slag and silica fume; and a polycarboxylate dispersant comprising a copolymer of an oxyalkylene-alkyl ether and an unsaturated dicarboxylic acid. The invention was purportedly based on the patentee's observation that the fluidity of conventional dispersants, such as sulfonated melamines and sulfonated naphthalenes was not sufficient to permit the substitution of beta gypsum for alpha gypsum in high strength flooring applications, a result permitted only by the use of the more expensive polycarboxylate dispersants comprising a copolymer of an oxyalkylene-alkyl ether and an unsaturated dicarboxylic acid. The patent also indicates that continuously calcined synthetic gypsum is equivalent to beta-calcined hemihydrate.